1. Field of the Invention
The invention relates to a motor control device that calculates a rotation speed of a motor.
2. Discussion of Background
A control device for an electric power steering system, which is described in U.S. Pat. No. 6,260,655 B1, calculates an estimated value of a rotation speed of a motor on the basis of a voltage value of the motor, which is measured by a voltage sensor, a current value of the motor, which is measured by a current sensor, and an estimated resistance value that is an estimated value of a resistance of the motor. When the control device has determined that the rotation of the motor is stopped, the control device calculates an estimated resistance value of the motor by dividing the voltage value of the motor by the current value of the motor.
The resistance of the motor fluctuates during rotation of the motor. On the other hand, the control device does not update the estimated resistance value during rotation of the motor. Therefore, there is a possibility that, during rotation of the motor, a deviation in the estimated resistance value that is used to calculate the rotation speed of the motor may increase. Therefore, there is a possibility that a deviation that is included in the estimated value of the rotation speed of the motor may also increase.
Here, the problem regarding the control device for an electric power steering system is referred to. However, a motor control device that calculates an estimated value of a rotation speed of a motor, such as a control device for an electro-hydraulic power steering system, also has the same problem.